In the fabrication of integrated optics chips from optical material wafer substrates, a proton exchange process forms low loss waveguides in the optical material wafer substrate. The optical material wafer substrate in one example comprises an optical quality lithium niobate (LiNbO3) wafer. In one example, the lithium niobate wafer is immersed in a melt of benzoic acid to initiate the proton exchange process. In another example, the lithium niobate wafer is immersed a melt of 1% to 4% lithium benzoate in benzoic acid. In yet another example, the lithium niobate wafer is exposed to benzoic acid vapors at an elevated temperature to initiate the proton exchange process.
The lithium niobate wafer is masked to define a pattern of waveguide structures. A diffusion process substitutes protons for lithium ions in a lattice of the lithium niobate wafer. The lithium niobate wafer may then be diced into chips, such as multifunctional integrated optics chips (“MIOC's”). Low optical loss is achieved in the waveguides by adjusting the temperature, time, and mole fraction of lithium benzoate in the melt. For waveguides formed with melt compositions containing less than 2.6% lithium benzoate (dependent on temperature) it may be necessary to thermally anneal the waveguides after the proton exchange process to achieve low loss in the waveguides.
Crystals of optical material wafers, such as the lithium niobate wafer, are subject to a pyroelectric effect. The pyroelectric effect causes a linear change in spontaneous polarization in the lithium niobate wafer as a function of temperature. The pyroelectric effect is the result of the movement of the lithium and niobium ions relative to the oxygen layers of the lithium niobate wafer. Since the lithium and niobium ions move only in a direction parallel to the Z-axis (e.g., principal or c-axis of the crystal), a potential difference is set up between the two Z-axis faces of the crystal. Since the pyroelectric tensor of a lithium niobate crystal is negative, cooling the lithium niobate crystal causes the +Z face to become positively charged. As one shortcoming, the static charges generated by the pyroelectric effect limit the performance of integrated optical devices over temperature. For example, the static charges generated by the pyroelectric effect interfere with electrical biasing and modulation schemes of the integrated optical devices. The static charges generated by the pyroelectric effect may even cause sparking in the integrated optical devices.
In attempt to mitigate the pyroelectric effect, conductive depositions and circuits have been added to the optical material wafers, such as the lithium niobate wafer, to bleed off pyroelectric static charges from the z-axis crystalline faces, where they arise due to temperature excursions of the device. The conductive depositions and circuits may prevent sparking and decrease relaxation times of the lithium niobate wafer. However, as another shortcoming, the conductive depositions and circuits may fail to prevent the potential gradient within the crystals of the lithium niobate wafer, most importantly in the vicinity of the waveguides.
Thus, a need exists for a fabrication process of integrated optics chips that serves to mitigate the pyroelectric effect in the integrated optics chips.